The present invention relates to an evaporation process and an evaporator.
Generally, in a multiple-effect evaporator, high-temperature steam introduced from an external source to the highest-temperature effect as a heat medium goes through the heat-transfer tubes to evaporate brine introduced around the tubes. Vapor generated on the tubes is utilized as the heat medium for the next effect. The above function is repeated sequentially down to the lowest effect and finally the vapor is condensed in a condensor. Thus, in each effect, the steam or vapor is utilized for evaporation only once. If vapor generated in the lowest effect is drawn with a steam ejector without being condensed and is mixed with the driving steam for reuse as a heat medium for the multiple-effect evaporator, the water production ratio, i.e., the ratio of distillate out-put to steam input, will be increased.
The heat-tranfer coefficient of heat-transfer tubes of the evaporator is improved by arranging more bundles of the tubes in an evaporation chamber so that vapor will flow through one tube bundle after another at a high velocity uniformly in every tube, although such arrangement costs a little higher.
There has conventionally been an evaporator of the vapor compression type having two separate bundles of horizontal heat-transfer tubes. Feed-liquid is sprayed over the tubes and the evaporated portion of the liquid is compressed by an engine-driven compressor before it is fed into the upper bundle of tubes for heat-exchange. Fluid in the upper tubes turns into the lower tubes from which condensate and non-condensible gases are discharged. The outer surface of the lower tubes are subjected to the high-temperature sprayed liquid as heated on the upper tubes, so that non-condensible gases passing through the lower tubes will not be fully cooled and must be discharged together with a large amount of steam. This increases the amount of gas discharge, necessitating a large-size gas extractor or a separate or extra vent condenser to minimize thermal losses.
These undesirable subjects will be further improved by simultaneously effecting the arrangement of separate bundles of heat-transfer tubes and the introduction of subcooled feed-liquid onto the top bundle of heat transfer tubes, then introducing high-pressure steam mixed with low-pressure vapor into the bottom bundle of heat-transfer tubes. However, if the mixed vapor is too hot, scales may form on the heat-transfer tubes. Generally, as steam of high-temperature and high-pressure adiabatically expands through a steam ejector, it becomes superheated steam of low-pressure and high-temperature, which, because it is too hot, should not be used for evaporation as it is even if mixed with vapor from the lowest-temperature effect.